Participants completed the euhydration condition first. They maintained habitual fluid intake for approximately 12 hours prior to testing, consuming 1.5-2.0 litres of water ad libitum during the day and night. On the morning of testing, a standardized breakfast was consumed along with 500 ml of water approximately two hours before testing. Participants were instructed not to urinate during the final 30 minutes prior to the session. Hydration status was verified using first-morning urine samples assessed via Armstrong's eight-point urine colour chart (target 1-2) and pre-test body mass measurements. Once classified as adequately hydrated, participants completed the d2 Attention Test, the Bruininks-Oseretsky Test of Motor Proficiency - Short Form (BOT-2 SF), and a structured football skill test. Attention and motor tests were conducted indoors, followed by the football skill test on an outdoor pitch. All testing was performed under supervision, with small participant groups to ensure st

After completing the euhydration condition and following a minimum 48-hour washout period, the same participants completed the hypohydration condition. Fluid intake was restricted beginning at 17:00 the day before testing, with no fluids overnight, and a low-fluid breakfast was provided on the morning of the session. Hypohydration was induced via a controlled treadmill walking protocol, with speed and duration individually adjusted to achieve 2-3% body mass loss. Participants were continuously monitored for heart rate, perceived exertion (Borg 6-20 scale), and localized muscle discomfort. Body mass was measured every 10 minutes, and fluid intake was not permitted during exercise. Hydration status was confirmed via pre- and post-exercise urine colour and body mass measurements, with urine strip tests as a secondary check. After a 15-minute seated recovery and consumption of low-water snacks, participants repeated the d2 Attention Test, BOT-2 SF, and football skill test under hypohydrat

Participants maintained their habitual fluid intake for 12 hours prior to testing, approximately 1.5-2.0 L ad libitum throughout the day and night, and consumed 500 mL of water with a standardized breakfast approximately two hours before the test. Participants were instructed to refrain from urinating during the 30 minutes preceding testing. Hydration status was verified using first-morning urine colour assessed on Armstrong's 8-point scale (target range 1-2) and pre-test body mass measurements. Cognitive, motor, and football skill performance were subsequently evaluated under these euhydrated conditions using the d2 Attention Test, BOT-2 SF, and a structured football skill test, respectively.

Body mass index (BMI) was calculated using height and body weight measurements obtained at baseline. Height was measured once at baseline using a standard stadiometer, and body weight was measured immediately prior to any exercise or fluid manipulation using a calibrated scale with ±50 g precision (InBody 120). BMI was calculated as body weight (kg) divided by height squared (m²).

Hydration status was assessed using first-morning urine samples (50-60 mL) collected prior to exercise and immediately following the exercise-induced dehydration protocol. Urine samples were analyzed using Armstrong's eight-point urine color chart. Urine color values of 1-2 indicated euhydration, whereas values of 5-6 indicated moderate hypohydration.

Exercise-induced body mass loss was used as an objective indicator of hydration status. Body weight was measured immediately before exercise and again within 5 minutes after completion of the treadmill protocol. Percentage hypohydration was calculated as the percentage change in body mass relative to baseline body weight. A body mass loss of 2-3% was considered indicative of significant hypohydration.

Perceived exertion was assessed using Borg's 6-20 Rating of Perceived Exertion (RPE) scale, and localized muscle discomfort was monitored through direct observation, particularly in the lower limbs. Ratings were recorded at regular 10-minute intervals during the treadmill protocol to ensure participant safety and to monitor subjective fatigue.

Selective attention, processing speed, concentration, and attentional stability were assessed using the d2 Attention Test. Primary outcome parameters included total number of items processed (TN), error percentage (%E), concentration performance (CP), fluctuation rate (FR), and net performance (TN-E).